The present invention relates to a method of manufacturing the substrate of GaAs compound semiconductor, in particular, to the method of manufacturing semi-insulating GaAs substrate in which the distribution of characteristics is uniform and which has high resistivity.
GaAs has a high electron mobility among the compound semiconductors of III-V groups and is being used broadly as a crystal substrate for of ultra high-speed integrated circuits and optoelectronic integrated circuits. This GaAs substrate is usually manufactured from a ingot obtained by the liquid encapsulated Czochralski method (hereinafter abbreviated as LEC method). However, the GaAs substrate made by LEC method has generally a large distribution of characteristics in the substrate (Japanese Journal of Applied Physics, vol. 21, No. 6, '82, p. L335-L337). For making the distribution of characteristics uniform, the ingot-annealing method wherein heat treatment at high temperature for a long time (800.degree.-1100.degree. C..times.2-48 hours) is given to the GaAs single crystal in the form of ingot is broadly put into practice (GaAs IC Wymposium, '83 IEEE, D. Rumsky et al). Namely, it is reported that, according to said ingot-annealing method, the uniformity of various characteristics, for example, the threshold voltage being important for IC is attained (Appl. Phys. Lett., 44, '84, p. 410, Miyazawa et al).
This ingot-annealing method is carried out in a way that, after heating the crystal for many hours, it is cooled at a rate as slow as 50.degree. C./hour in order not to cause strains in the crystal.
For the substrates to be used for LSI and IC, high resistivity is also important together with the uniformity of characteristics. This is so since, if the resistivity is high, the condition of separation between elemnts becomes good and the degree of integration can also be made large. With the substrate given the ingot-annealing, however, the uniformity of characteristics is improved as described above, but the resistivity decreases resulting in causing a problem in the leakage current between elements or that from the active layer to the substrate.
As a result of extensive investigations in view of this situation, a method of manufacturing the substrate of GaAs compound semiconductor capable of raising the resistivity of substrate has been developed by the invention.